What I've read about the TerraLUX upgrade has been uniformly positive. Some time ago I bought the Mag Instruments LED upgrade for our 3xD Maglites. While I doubt it's as bright as the TerraLUX emitter, it's worked very well. The beam is ringy and full of artifacts but the lights have stood up to unconscionable abuse while remaining watertight and giving outstanding battery life.

I wouldn't buy a 3xD Maglite now (too many good lights out there that outperform it in much smaller form-factors) but the LED upgrade made them worth keeping.